The invention relates to a circuit for recognizing data collisions in an optical data bus which occur when a subscriber in addition to a transmitting subscriber is also transmitting data. The invention further relates to a circuit for recognizing the data-free status of the bus.
Bus network structures have been gaining increasing acceptance in comparison to point-to-point connections, for the call setup of a plurality of subscribers (computers, terminals, etc.). A significant requirement for an ordered function sequence on the bus is the prevention of more than one subscriber from simultaneously transmitting data on the bus line. In synchronously functioning systems, this requirement for an ordered function sequence can be fulfilled by means of a defined allocation of time slots for individual subscribers. Asynchronous buses are more flexible, and in many applications, enable a higher data throughput. An example of an asynchronous access method is the "Carrier-sense-multiple-access scheme with collision detection" (CSMA/CD) which is applied and described in the Ethernet (See Comm. of the ACM, July 1976, Vol. 19, No. 7, pp. 395-404, incorporated herein by reference).
In the asynchronous method, the subscriber wishing to send, for example, computer data, must determine at the point in time at which it wishes to send whether data of at least one other computer is arriving at the receiver allocated to it. When no data is arriving, it assumes that the bus is free of data and thus a so-called transmission medium idle status exists. It now begins to transmit its data onto the bus. It can occur during the time in which this computer sends its data that data from at least one other computer arrives at the receiving computers. This must be perceived in the sending computers as a collision status.